Scroll type fluid displacement apparatus are well known in the prior art. For example, U.S. Pat. No. 801,182 discloses a scroll type fluid displacement apparatus including two scroll members, each having a circular end plate and a spiroidal or involute spiral element. These scroll members are maintained angularly and radially offset so that both spiral elements interfit to make a plurality of line contacts between their spiral curved surfaces to thereby seal off and define at least one pair of fluid pockets. The relative orbital motion of the two scroll members shifts the line contacts along the spiral curved surfaces and, therefore, the fluid pockets change in volume. The volume of the fluid pockets increases or decreases depending on the direction of the orbiting motion. Therefore, scroll type fluid displacement apparatus are applicable to compress, expand or pump fluids. For the sake of convenience, the discussion which follows deals only with a scroll type device used as a compressor.
In comparison with conventional compressors of the piston type, a scroll type compressor has certain advantages, such as fewer parts and continuous compression of fluid. However, there have been several problems, primarily in the sealing of the fluid pockets. Sealing of the fluid pockets must be sufficiently maintained at the axial and radial interfaces in a scroll type compressor, because the fluid pockets are defined by the line contacts between the interfitting spiral elements and axial contact between the axial end surfaces of the spiral elements and the inner end surfaces of the end plates.
The principles of operation of a scroll compressor will be described with reference to FIGS. 6a-6d. FIGS. 6a-6d schematically illustrate the relative movement of interfitting spiral elements to compress fluid, and may be considered to be end views of a compressor wherein the end plates are removed and only the spiral elements are shown. As illustrated in FIG. 6a, the orbiting spiral element 1 and the fixed spiral element 2 make four line contacts at four points A-D to define fluid pockets 3a and 3b. A part of fluid pockets 3a and 3b is defined between line contacts D-C and line contacts A-B, as shown by the dotted regions; and also by the contact of the axial ends of spiral elements 1 and 2 with the end plates from which these spiral elements extend. When orbiting spiral element 1 is moved in relation to fixed spiral element 2 center 0' or orbiting spiral element 1 revolves around center 0 of fixed spiral element 2 with a radius of 0-0', while the rotation of orbiting spiral element 1 is prevented. The pair of fluid pockets 3a and 3b thus shift angularly and radially towards the center of the interfitting spiral elements with the volume of each fluid pocket 3a and 3b being gradually reduced, as shown in FIGS. 6a-6d. The fluid in each pocket is thereby compressed.
Accordingly, if circular end plates are disposed on, and sealed to, the axial facing ends of spiral elements 1 and 2, respectively, and if one of the end plates is provided with a discharge port 4 at the center thereof as shown in FIG. 6, fluid is taken into the fluid pockets at the radial outer portion and is discharged from the discharge port 4 after compression.
In this arrangement, as mentioned above, the two scrolls are maintained angularly offset by 180.degree. to securely define the line contacts. However, if the angular relationship between the scrolls is moved from this formal arrangement, because of inaccuracy in the manufacturing or assembly process, the line contacts break to a degree, thereby adversely effecting the efficiency of the compressor.